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The objective of peripheral nerve location by electrical stimulation is
to elicit a targeted motor response by a block needle coupled to a
(square-wave) current generator (ie, nerve stimulator). The stimulator
provides a stream of square-wave pulses, typically at a frequency (f) of 1
to 2 Hz. Ability to elicit designated motor responses below threshold
current levels that have been empirically associated with high success rates
indicates immediate proximity to the nerve.
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The ability to electrically stimulate a peripheral nerve or neural
plexus depends on:
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- 1. Electric current amplitude (I), ie, the amperage applied to the
stimulator electrode or needle
- 2. Pulse duration or width of the square wave of current generated by the
pulse oximeter
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And is inversely proportional to:
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- 3. The distance between the stimulating electrode and the nerve
- 4. Tissue electrical impedance (mostly resistance) of the tissues that lie
between and around the electrode and the targeted nerve or nerves
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Current Amplitude (Amperage)
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Use of higher amperage (eg, 2–5 mA) to stimulate peripheral nerves
allows one to elicit a motor response at a greater distance from the nerve.
As an electrode (the needle) approaches the nerve, motor response to
electrical stimulation can be achieved at lower amperage (Figure 45–1).
This is governed by Coulomb's law equation,
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where E = required stimulating current, K = constant,
Q = minimal required stimulation current, and r = distance between electrode and nerve.
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Empirically, motor response to stimulation with current below 0.5 mA
with pulse duration of 0.1 ms signifies that the needle's tip is
sufficiently close to the nerve to translate to a high block success rate.
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According to Coulomb's law, if a motor response can be elicited at very
low amperage (<0.5 mp), then the stimulating electrode must be very
close to the nerve. Stimulation at very low amperage maximizes specificity.
In contrast, using a higher amperage (eg, 2–5 mA), maximizes sensitivity.
This principle is used when monitoring the neuromuscular function by
cutaneous electrodes and comparably very high (≈50 mA)
currents during general anesthesia. Here, specificity of electrode location
relative to the nerve is of less importance. For peripheral nerve location,
2- to 5-mA currents increase sensitivity, providing clues at a distance, but
ultimate specificity is achieved by successful stimulation at very low
current (<0.5 mA). However, it should be noted that using current
of high intensity has practical disadvantages in that (1) it is associated
with patient discomfort and (2) higher current intensity (eg, >1.0
mA) is sufficient to elicit direct muscle stimulation, which may produce
confusing twitches of the local muscles. For these reasons, many nerve
localization procedures are best initiated using relatively lower initial
stimulating current (eg, <1.5 mA).
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The surface area of the conductive electrode and its conductance are very
important in nerve stimulation according to Ohm's law: I=V/R, where I = current flow (amperage), V = potential difference (voltage),
and R = resistance (ohms).
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The nerve stimulator varies current flow by altering the voltage. Most
modern nerve stimulators are constant-current generators that automatically
adjust the voltage output to maintain the set current flow despite changes
in tissue resistance (within a certain range specified by the electrical
design of the nerve stimulator).
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Resistance, on the other hand, is related to the conductive area of the
electrode by the equation R = ρ L/A, where R = electrical resistance, ρ = tissue resistivity,
and A = conductive area. An example of the clinical importance of this
principle is the use of defibrillating paddles. Defibrillation paddles are
characterized by a large surface area to minimize impedance or resistance.
By contrast, the microelectrode tip of a shielded block needle serves to
maximize resistance outside of the microconductive area. This ensures that
the electrode tip must be very close to the nerve if a motor response is to
be elicited, thereby enhancing specificity.
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If a motor response can be elicited at (1) very low amperage and, by
Ohm's law, with a (2) microelectrode (small conductive area), then the
stimulator electrode must be very close to the nerve. This phenomenon has
led to the clinical use of needles with electrically insulated shafts to
ensure specificity of location to the microelectrode (needle's tip) relative
to the targeted nerve. Bashein and colleagues4 looked at
the difference in the relative dispersion of current between electrically
shielded and unshielded needles. Indeed, the ability to elicit a motor
response to electrical stimulation following the initial elicitation of a
mechanical paresthesia differs from 40%, using noninsulated needles, to
10% with insulated needles.5 The 30% increase in the
ability to cause motor nerve stimulation with a noninsulated needle compared
with an insulated needle can be explained by the difference in current
dispersion between the two needles (see Chapter 5, Electrophysiology of
Nerve Stimulation, for more information).
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Electrical Pulse Duration
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Electrical pulse duration refers to the duration of the periodic pulsed
square wave generated by the nerve stimulator. For the purpose of nerve
localization, short pulse duration ranging between 0.05 and 1 ms is
typically used in clinical practice, with 0.1 ms being most common.
Increasing electrical pulse duration increases the total flow of electrons
(electrical energy) proportional to the calculated area under the curve
(Figure 45–2). Increasing the duration of the electrical pulse
therefore results in increased ability to stimulate the nerve without
changing other variables. Similar to current flow (amperage), higher pulse
durations of 0.3 to 1.0 ms also result in enhanced sensitivity for
transcutaneous or initial invasive prelocation of the nerve. By contrast, by
using a lower pulse duration, specificity is maximized. This principle has
been demonstrated clinically when higher current amplitude (amperage) was
needed to elicit a motor response at lower pulse duration (Figure 45–3).6
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Tissue Electrical Impedance
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Tissue impedance is a function of electrical resistance, capacitance,
and inductance of the biologic tissue. In general, the higher the
water–lipid ratio of the tissue, the lower the electrical impedance, or
conversely, the higher the tissue conductance, R = ρ L/A, where ρ is the tissue resistivity.
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Condensing the tissues by indenting the overlying skin, adipose, etc
toward the nerve serves to decrease electrical impedance, increasing
conductance.
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Electrical Pulse Frequency
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The frequency (f) of the square-wave electrical pulse generated by
the nerve stimulator is typically set at 1 or 2 Hz. Increasing frequency to
2 Hz gives more rapid feedback with little added discomfort to the patient.
Frequency must be sufficiently low so as to allow time for the relaxation
phase period following depolarization. For example, frequency of 50 Hz
causes sustained tetanus and is extremely painful and is therefore
unacceptable for locating peripheral nerves for regional blockade. In
addition, stimulation at frequencies greater than 3 Hz results in a loss of
specificity; ie, motor response may be indistinguishable from a muscle
fasciculations.